The invention relates to joint prostheses. More particularly, the invention is directed to components of knee joint prostheses that have a tibial bearing insert which can rotate and translate with respect to the tibial tray upon which it is mounted.
Joint replacement surgery is quite common and enables many individuals to function normally when otherwise it would not be possible to do so. Artificial joints are normally composed of metallic, ceramic and/or plastic components that are fixed to existing bone.
Knee arthoplasty is a well known surgical procedure by which a diseased and/or damaged natural knee joint is replaced with a prosthetic knee joint. Typical knee protheses include a femoral component, a patella component, a tibial tray or plateau, and a tibial bearing insert. The femoral component generally includes a pair of laterally spaced apart condylar portions, the distal surfaces of which articulate with complementary condylar elements formed in a tibial bearing insert.
Typically, the tibial tray is mounted within the tibia of a patient. The tibial bearing insert, which is usually made of ultra high molecular weight polyethylene (UHMWPE) is mounted upon the superior surface of the tibial tray. Load and stress are placed upon the knee prosthesis, and particularly on the tibial bearing insert, during normal daily use. These forces may lead to the displacement or dislocation of the insert from the tibial tray. To accommodate these forces, and to reduce the chances for dislocation, some tibial components of knee prostheses have been designed to allow rotation of the tibial bearing insert relative to the proximal or superior surface of the tibial tray, about the longitudinal axis of the prosthesis. Such rotation, when controlled, can increase the contact area between the femoral condyles and the tibial bearing insert throughout the range of knee motion, thus reducing stress on the tibial bearing insert.
Some knee prostheses that allow for rotation of the tibial bearing insert also permit translation, or roll back, of the tibial bearing insert in the anterior-posterior direction.
There is often a trade-off between the longevity of a knee prosthesis and the overall performance or kinematics of the prosthesis. It has been difficult to optimize both longevity and kinematics. Rotatable knee prostheses have provided some benefits. However, some rotatable knee prostheses that permit translation of the tibial bearing insert in the medial-lateral and anterior-posterior directions can yield some degree of anterior-posterior instability. This problem is overcome, in some instances, by posterior stabilized prostheses. However, in such designs, movement of the tibial bearing insert in the anterior-posterior direction, or roll back, can place undesirable levels of stress on the stabilizing post of the prosthesis.
Despite the benefits of existing designs for knee joint prostheses having a rotatable tibial component, there remains a need for prostheses that still reduce stress, provide optimal roll back and maintain optimal contact of the tibial and femoral components to accommodate the stresses placed upon the knee.